That's already done today. Most modern codecs support variable bitrate encoding so more of the data budget can be given to high complexity scenes. Source video can also be processed multiple times with varied parameters and that output then compared structurally to the source to find the parameters that best encode the scene. This is beyond the more typical multi pass encoding where a first pass over the source just provides some encoding hints for the second pass. It takes several times longer to encode (though is embarrassingly parallel) but the output ends up higher quality for a given data rate than a more naïve approach.